The present invention relates to a control system for a continuously variable transmission.
Laying-open Japanese patent application No. 58-170958 discloses a method of controlling a reduction ratio established in a continuously variable transmission including a transmission V-belt tensioned between a driver pulley and a follower pulley. According to this known method, an actuator for effecting a shift in reduction ratio is activated upon receiving a brake signal which is generated when the driver depressing a foot brake to initiate downshift toward the largest reduction ratio.
Laying-open Japanese patent application No. 58-170959 discloses another method of controlling a reduction ratio established in a continuously variable transmission. According to this known method, an actuator for effecting a shift in reduction ratio is activated upon detecting the fully closed state of a throttle valve to initiate downshift toward the largest reduction ratio.
According to the known method mentioned above, since the downshift operation starts immediately after receiving the brake signal or upon detecting the fully closed state of the throttle valve, the effective engine brake running is obtained quickly in the case of a continuously variable transmission that has a releatively slow shift speed. However, since the downshift operation is initiated upon anticipating the need for engine brake running, the transmission starts shifting down if the driver steps on the brake pedal without demanding the engine brake effect. In this case, the vehicle's performance fails to correspond to the driver's feeling.
Another problem encountered in the known control system carrying out the above mentioned method is that if, after quick brake to such an extent that the vehicle's driving wheels are locked to stop rotation of the transmission output shaft, the driver depresses the accelerator pedal, the transmission V-belt slips, failing to transmit torque, causing the V-belt to wear badly to result in shortened operating life. This is because the speed at which the transmission downshifts is not as high as the speed at which the quick brake operation progresses and besides the shifting response is very poor after the follower pulley that is rotatable with the transmission output shaft has stopped its rotation. This brings about the situation where the hydraulic fluid is being discharged from the driver pulley and sufficiently high friction does not exist between the transmission V-belt and the pulley when the accelerator pedal is depressed, causing the V-belt to slip.
An object of the present invention is to improve a control system for a continuously variable transmission such that immediately after quick brake has been released, a transmission V-belt is held tensioned between driver and follower pulleys and thus can transmit torque without any slip between the pulleys and the V-belt when an engine torque is applied to the driver pulley again.
Another object of the present invention is to improve a control system for a continuously variable transmission such that the transmission downshifts at a satisfactorily high speed during quick brake operation.